Field of the Invention
This invention relates to actuators for electrically operating switches in electric power circuits, and to electric power switches incorporating such actuators. The actuator is a flux transfer shunt trip actuator in which a coil is pulsed to generate a field which bucks the holding force generated by permanent magnets so that the spring drives the actuator to an actuated position to trip the operating mechanism of the electric power switch. More particularly, it relates to an arrangement for adjusting the response of the flux transfer shunt trip actuator.
Background Information
Electric power switches, such as for example, circuit breakers, transfers switches, network protectors, disconnects, and the like, typically have a stored energy powered operating mechanism that is tripped to rapidly open the power contacts of the switch. It is common to have electrically actuated devices actuating or tripping the operating mechanism. Even in the case of a circuit breaker with a thermal-magnetic trip unit, an electrically actuated device is often also provided to alternatively actuate the operating mechanism from a remote location or for other conditions, such as under voltage, loss of phase, or off frequency. In many cases, an electronic trip circuit is used instead of a thermal-magnetic mechanism for overcurrent protection.
It is becoming wide spread for the electronic circuits now used to actuate these trip devices to generate logic levels signals which have insufficient energy to actuate solenoids previously used for these purposes. A low energy device which has been developed to trip the operating mechanism of an electric power switch in response to logic level signals, is the flux transfer shunt trip actuator. These devices utilize a spring to bias a plunger to an actuated position. Permanent magnets generate sufficient magnetic force to override the actuating force generated by this spring and hold the plunger in an unactuated, typically a retracted, position. The actuating signal is applied to a coil to generate an electromagnetic force which bucks the force generated by the permanent magnets. With the permanent magnet field at least partially cancelled, the spring actuates the plunger. A short duration pulse applied to the coil is sufficient to actuate the plunger and trip the operating mechanism of the switch.
While these flux transfer shunt trip devices are very effective, slight variations in the strength of the magnets, springs, or even plating thicknesses can affect the current required to release the plunger. U.S. Pat. No. 5,886,605 discloses a flux transfer shunt trip actuator which incorporates an arrangement for calibrating the device. In this patented actuator, the spring biasing the plunger to the tripped position is mounted inside the cylindrical plunger. A screw threaded into the free end of the plunger bears against the spring to adjust the spring preload. While this is effective for calibrating the device, it results in changing the effective length of the plunger, and therefore, varies the spacing between the plunger and a trip button on the operating mechanism of the electric power switch.
Accordingly, there is room for improvement in flux transfer shunt trip actuators and electric power switch incorporating them.
In fact, there is a need for a flux transfer shunt trip actuator and an electric power switch incorporating such an actuator, which provides an improved arrangement for calibrating the actuator.
More particularly, there is a need for such an actuator and electric power switch incorporating the actuator, in which the actuator can be calibrated without affecting the spacing between the actuator plunger and the operating mechanism of the electric power switch.
There is a more specific need, for an improved flux transfer shunt trip device and an electric power switch incorporating the actuator, in which the preload on a spring biasing the actuator plunger can be adjusted without affecting the positioning or length of the plunger.